Seismic imaging involves acoustic waves emitted in the sea or on the surface and which propagate through a medium, undergoing therein, among other things, reflections and refractions. These waves are then recorded in the form of signals by means of sensors such as geophones. These signals are in the form of seismic traces representing the variations in amplitude of a wave as a function of time.
To monitor the change over time in a geological medium, the seismic imaging proceeds by successive acquisitions of seismic data with the same sources and detectors. It is usual to refer to such monitorings by speaking of “quadridimensional or 4D seismic imaging”. Such monitoring proves to be useful for surveying hydrocarbon deposits. It is for example desirable to know the influence of exploitation on the properties of oil or gas reservoirs, with a view to preventing any collapses.
Exploitation of a deposit causes a reduction in the quantity of hydrocarbons present in a reservoir, which may affect the composition of the surrounding rocks and disturb the speeds of propagation of seismic waves passing through this modified medium. These changes have an effect at the detectors through different recorded arrival times, inviting the updating of the velocity model used for modelling the propagation of the seismic waves in the imaged medium and the construction of a representation of this medium.
However, such monitoring involves digital processing operations that may be lengthy because of the large volume of data accumulated. Moreover, the monitoring generally suffers from lack of precision because of the difficulty existing in identifying the small variations that occur between two acquisitions. A compromise between speed, simplicity of processing and precision is therefore in particular sought.
The seismic imaging method by means of which a physical model is obtained from seismic data is a so-called inversion method. Various inversion methods have been proposed to implement 4D imaging.
One simplistic technique consists in considering all the interfaces reflecting the seismic waves, also referred to as reflectors, as being planar reflectors in the imaged medium, and carrying out the imaging in one dimension, referred to as 1D imaging. This method has the advantage of being quick, but very inadequate since the reliability of the image degrades rapidly with depth. In addition, it provides information only along one axis whereas a representation in three dimensions is sought. It is possible, by way of example of methods using 1D imaging techniques, to consult the patents FR 10/55945 and FR 10/57508, defining respectively a digital method for modelling changes in a stratified model and a method for extrapolating a 3D model from 1D measurements along a well.
An improved version of 1D imaging consists in orienting the detection axis perpendicular to an interface or a set of interfaces with a known non-zero inclination. This technique does however prove to be ineffective for detecting complex structures, for example reservoirs in the form of domes, or imaging with sufficient precision mediums having reflectors of variable inclination.
Another approach consists in solving the complete problem of inversion in three dimensions from a plurality of seismic signals emitted in the three directions in space. This so-called “full-wave inversion” technique makes it possible to cross-check redundant information and to obtain a more realistic three-dimensional image of the medium. However, it proves to be extremely expensive in terms of time. The slowness of this technique in practice makes it applicable only at low frequencies of no more than 15 Hz. In the end, this technique is therefore not sensitive to small changes in the medium occurring at scales lower than the wavelength of the seismic waves used.
Consequently, an inversion technique is sought suitable for 4D imaging that is rapid while offering sufficient precision and resolution to be able to detect reservoirs having complex structures, for example in the form of a dome.